Multicolored image forming method and apparatus therefor

ABSTRACT

An image forming method and an apparatus therefor for forming a reproduced color image on a recording medium by repeating a process of forming on a latent image carrier an electrostatic latent image corresponding to a document image through an image exposure, depositing one of at least yellow, magenta and cyan toners on the electrostatic latent image, wherein a color which is read as a mixture of magenta and cyan is deposited as magenta or cyan according to the proportion of mixing of magenta and cyan in the mixed color.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an electrophotographicprocess and, more particularly, to an image forming method and a devicetherefor designed to reproduce a multi-colored image by the use of theelectrophotographic process.

2. Description of the Prior Art

With increase in number of available functions in a copying machine andalso with increased applications thereof, the availability is expectedof a copying machine capable of making a copy of a multi-colored imagecorresponding in color to the multi-colored original document. However,the copying machine currently available in the market is generally of atype capable of making copies on monochromatic basis.

Accordingly, if attempt is made to make this kind of image formingmachine capable o making multi-colored copies, the machine has to beequipped with a number of developing units equal to the number of colorsdesired to be reproduced and, at the same time, the machine is requiredto have a capability of transferring and fixing a corresponding numberof powder images onto one and the same copying sheet.

However, when it comes to the copying machine having a resolution ofreproducing seven to eight colors, rather than two to three colors, thespace in the machine occupied by the plural developing units will beconsiderably increased and, accordingly, not only the latent imagecarrier, but also the machine itself will become considerably bulky andexpensive. Also, since the copying sheet is thermally affected each timeit is passed through a fixing unit, the copying sheet when passedthrough the fixing unit several times will be curled to such an extentas to result in a displaced reproduction of the image on the copyingsheet.

Also, in a multi-color development, a problem associated with colordistortion tends to occur when a color reproduction is attempted withthe use of two or more kinds of toner material. Although this problemwill not be noticeable if combined with yellow, a combination of magentaand cyan tends to result in the noticeable color distortion.

SUMMARY OF THE INVENTION

Accordingly, the present invention is intended to provide an imageforming method and an apparatus therefor wherein a minimized number ofdeveloping units are employed to form an image in a number of colors asmany as possible.

Another important object of the present invention is to provide an imageforming method and an apparatus therefor of the type referred to above,wherein no color distortion will occur even when a combination ofmagenta and cyan is employed.

According to the present invention, the above described objects can beaccomplished by providing an image forming method which comprises aprimary transfer step of causing a toner image on an electrostaticlatent image carrier onto a transfer belt and a secondary transfer stepof transferring the toner image, which has been transferred onto thetransfer belt, onto a copying sheet, wherein cyan or magenta toner istransferred from the latent image carrier onto an identical area of thetransfer belt in overlapping relationship and, subsequently theoverlapped toner are transferred onto the copying sheet.

According to the above image forming method, for example, cyan toner istransferred onto the transfer belt so as to overlap the yellow tonerwhich has been transferred onto the transfer belt during the executionof the primary transfer step. At this time, since the cyan toner istransferred so as to overlay the yellow toner, the transfer efficiencyof the cyan toner will be lowered as compared with that of the yellowtoner.

Subsequently, both of the yellow and cyan toner overlapped with eachother on the transfer belt are transferred onto the copying sheet duringthe execution of the secondary transfer step. At this time, althoughalmost of the cyan toner positioned uppermost on the transfer belt canbe transferred onto the copying sheet, the amount of the yellow tonertransferred onto the copying sheet will be smaller than that of the cyantoner because the efficiency at which the yellow toner positionedbeneath the cyan toner is considerably reduced.

Accordingly, looking at the copying sheet, the cyan color is moreconspicuous than the yellow color and is perceived as a green image.

On the other hand, if the sequence of transfer is reversed, that is, thetransfer of the cyan toner is followed by the transfer of the yellowtoner, the yellow toner will be more conspicuous than the cyan toner andwill be perceived as a light green image.

Similarly, by combining the yellow toner and the magenta toner and bychanging the sequence of development, the red or orange color which is acolor intermediate between the yellow and the magenta ca be reproduced.

In other words, by using three colors, that is, yellow, cyan andmagenta, green, light green, red and orange which are all intermediatecolors can be accomplished other than those three colors.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction with apreferred embodiment thereof with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic side sectional view of a copying machine;

FIG. 2 is a graph illustrating spectral reflectivities of rays of lightradiated to images of different colors;

FIG. 3 is a flowchart showing a process of color discrimination;

FIGS. 4 to 9 are flowcharts showing a multi-color image forming process,wherein FIGS. 4 to 8 illustrate first to fifth steps and FIG. 9illustrates a monochromatic step;

FIG. 10 is a diagram showing an image on an original document; ,/

FIGS. 11 to 15 are diagrams showing an image forming process, whereinFIGS. 11(a) to 11(c) illustrate a first step, FIGS. 12(a) to 12(c)illustrate a second step, FIGS. 13(a) and 13(b) illustrate a third step,FIGS. 14(a) and 14(b) illustrate a fourth step, and FIGS. 15(a) and15(b) illustrate a fifth step;

FIG. 16 is a diagram showing the condition in which secondarilytransferred toner adheres to a copying sheet; and

FIG. 17 is a diagram showing an image formed through a process of thefirst to fifth steps.

DETAILED DESCRIPTION OF THE EMBODIMENT I. Structure of Copying Machine

(a) Photoreceptor Unit

FIG. 1 illustrates, in schematic side sectional representation, acopying machine which is used to practice a multi-color image formingmethod according to the present invention. As shown therein, aphotoreceptor drum 3 is supported at a generally central portion of thecopying machine housing 1 for rotation in one direction shown by thearrow a. In the vicinity of the outer periphery of the photoreceptordrum 3, an electrostatic charger 4, an editing eraser 5, a developingdevice 6, a transfer device 11, a cleaning unit 22 and a main eraser 23are disposed so that during one complete rotation of the photoreceptordrum 3 the latter can sequentially pass from the electrostatic charger 4towards the main eraser 23.

The editing eraser 5 comprises a plurality of light emitting elementsarranged in a multi-stage fashion within a holder disposed so as toextend parallel to the axis of rotation of the photoreceptor drum 3. Theediting eraser 5 is so designed that the timings at which the lightemitting elements of the editing eraser 5 are selectively energized ordeenergized can be controlled.

The developing device 6 comprises four developer units 7, 8, 9 and 10and is supported for movement up and down, as shown by the arrows b andb', through a plurality of operative positions at which one of thedeveloper units can confronts the photoreceptor drum 3 for applying adeveloping material (toner) onto the outer peripheral surface of thephotoreceptor drum 3. So far illustrated, the developer units 7 to 10accommodate therein respective masses of yellow toner Ty, magenta tonerTm, cyan toner Tc and black toner Tbk. It is to be noted that, althoughthe developing device 6 has been shown and described as supported formovement up and down, the present invention may not be limited thereto,but may be applicable where toners of different colors can beselectively applied onto the outer peripheral surface of thephotoreceptor drum 3.

(b) Transfer Device

The transfer device 11 includes an endless belt 15 trained aroundsupport rollers 12, 13 and 14 each extending parallel to the axis ofrotation of the photoreceptor drum 3 so that the endless belt 15 can bedriven in one direction shown by the arrow c and in a directionconforming to the direction of rotation of the photoreceptor drum 3.This endless belt 15 is adapted to receive and retain the toner, whichhas been applied onto the photoreceptor drum 3 and subsequentlytransferred onto such belt 15, and is provided with a dielectric bodysuch as, for example, polyethylene, on a surface of an electroconductivesubstrate made of electroconductive polyester containing carbon resin orthe like.

A pressing roller 16 is disposed between the support rollers 12 and 13for movement between retracted and urging positions; said pressingroller 16 when in the urging position urging a portion of the endlessbelt 15 between the support rollers 12 and 13 so as to contact thephotoreceptor drum 3, but said pressing roller 16 when in the retractedposition allowing that portion of the endless belt 15 to be separatedaway from the photoreceptor drum 3.

Positioned inwardly in the vicinity of the endless belt 15 and betweenthe support rollers 13 and 14 is a guide plate 18 cooperable with acleaning unit 19, a charge erasing charger 20 and an electrostaticcharger (SCOTRON charger) 21 all disposed on one side of a portion ofthe endless belt between the support rollers 13 and 14 opposite to theguide plate 18. A secondary transfer charger 24 and a separating charger25 adjoining the secondary transfer charger 24 are positioned below thesupport roller 14 so as to confront the endless belt 15.

(c) Optical System

The optical system is generally identified by 27 and positioned in anupper portion of the machine housing 1. This optical system 27 includesa first slider 28 on which an illuminator lamp 29, a color detector 30and a first reflecting mirror 31 are mounted. The first slider 28 issupported for movement immediately below and along a transparentdocument support 26 in a direction shown by the arrow d for scanning anoriginal document placed on the document support 26. The color detector30 is utilized to detect a color of the document on the document support26 by receiving rays of light which have been emitted from theilluminator lamp 29 and subsequently reflected from the document, thestructure and the function of which detector 30 will be described indetail later.

The optical system 27 also includes a second slider 32 positioned behindthe first slider 28 with respect to the direction of movement of thefirst slider 28, said second slider 32 having second and thirdreflecting mirrors 33 and 34 mounted thereon. The second slider 32 issupported for movement in a direction shown by the arrow d insynchronism with the movement of the first slider 28, but at a speedhalf the speed of movement of the first slider 28. The optical system 27further includes a fixedly supported fourth reflecting mirror 36, a lensassembly 35 positioned between the second slider 32 and the fourthreflecting mirror 36, a filter assembly 38 positioned between the lensassembly 35 and the fourth reflecting mirror 36, and a fifth reflectingmirror 37 positioned immediately above the photoreceptor drum 3.

(d) Filter Assembly

The filter assembly 38 comprises four filters, that is, a yellow filterFy, a magenta filter Fm, cyan filter Fc, and a black filter Fbk, whichare adapted to be selectively brought one at a time into the path oftravel of the rays of light from the lens assembly 35 towards the fourthreflecting mirror 36.

The yellow filter Fy is used to absorb the light which has been radiatedto a document image of a color containing a yellow component.Accordingly, as shown in the spectral reflectivity characteristic ofFIG. 2, when the yellow filter Fy is brought into the path of travel ofthe light rays, reflected components of the light which has beenradiated to images of yellow (Y), green (G) and red (R) colors can beabsorbed. In other words, the amount of rays of light radiated to theimages of the yellow color and other colors is lowered to a value whichmay be about equal to the amount of reflected component of the lightwhich is radiated to an image of black color.

Similarly, when the magenta filter Fm is brought into the path of travelof the light rays, the amount of light radiated to images of magenta(M), orange (OR) and red (R) colors will be lowered to a value aboutequal to the amount of reflected component of the light which isradiated to an image of black color, and when the cyan filter Fc isbrought into the path of travel of the light rays, the amount of lightradiated to images of cyan (C), light green (LG) and green (G) colorswill be lowered to a value about equal to the amount of reflectedcomponent of the light which is radiated to an image of black color.

The black filter Fbk is used to lower the amount of light radiated tothe image of black color down to a value equal to or smaller than apredetermined amount.

(e) Sheet Supply and Transport System

Within a lower portion of the machine housing 1, a sheet supply andtransport system is disposed, which comprises a sheet supply device 40including a first supply unit 41, a second supply unit 42 and a manualfeed unit 43.

Copying sheets 100 stacked in the first supply unit 41 are supplied oneby one by a supply roller 44 by way of a transport roller pair 45towards a secondary transfer station; copying sheets 100 stacked in thesecond supply unit 42 are supplied one by one by a supply roller 47towards the secondary transfer station; and a copying sheet 100 manuallyinserted in the manual feed unit 43 is supplied by a transport rollerpair 45 towards the secondary transfer station. The copying sheet sosupplied from one of the units 41, 42 and 43 is supplied to thesecondary transfer station, defined at a region where the transfer belt15 confronts the secondary transfer charger 24, after the timing atwhich the copying sheet 100 is actually supplied to the secondarytransfer station has been controlled by a timing roller 46. The copyingsheet 100 which has been passed through the secondary transfer stationis subsequently supplied by means of an endless transport belt 48towards the fixing unit 49 and then discharged onto a receiving tray 50.

II. Copying Operation

A fundamental operation of the copying machine of the above describedconstruction will now be described with reference to FIG. 1.

Assuming that a PRINT switch is switched on while the document (notshown) has been placed on the document support 26, a main drive motor 2is powered to rotate the photoreceptor drum 3 in the direction shown bythe arrow a and, substantially at the same time, the electrostaticcharger 4 is operated to electrostatically charge the outer peripheralsurface of the photoreceptor drum 3 to a predetermine potential.

On the other hand, in the optical system 27, the first and secondsliders 28 and 32 are driven in the direction shown by the arrow d withthe illuminator lamp 29 scanning the document on the document support26. Rays of light reflected from the document are, after having beenreflected by the reflecting mirrors 31, 33 and 34, subsequently passedthrough the lens assembly 35 and then through the filter 38, and againreflected by the reflecting mirrors 36 and 37, projected onto thephotoreceptor drum 3 at an exposure station to form an electrostaticlatent image on an imaging portion of the photoreceptor drum 3 incomplemental relationship to the image of the document.

As the photoreceptor drum 3 being rotated in the direction shown by thearrow a passes in front of the editing eraser 5, that portion of theelectrostatic charge built up on the photoreceptor drum which exteriorlysurrounds the imaging area of the photoreceptor drum where theelectrostatic latent image has been formed is erased by the editingeraser 5 by radiating rays of light thereto. It is to be noted that,where a multi-color image forming mode is selected as will be describedlater, an electrostatic charge of an image corresponding to apredetermined color is also erased by the editing eraser 5.

Subsequently, at a developing station, the electrostatic latent image onthe photoreceptor drum 3 is brought into alignment with the developingdevice 6 at which toner material is applied from a predetermined one ofthe developer units over the electrostatic latent image to form avisible powder image.

On the other hand, in the transfer device 11, based on the drive of themain drive motor 2, the pressing roller 16 is moved to the urgingposition a shown in FIG. 1 to urge that portion of the endless transferbelt 15 between the support rollers 11 and 12 so as to lightly contactthe photoreceptor drum 3 and, at the same time and during the continuedmovement of the endless transfer belt 15 in the direction shown by thearrow c, an electrostatic charge is applied uniformly over the endlesstransfer belt 15 from the electrostatic charger 21. It is to be notedthat the speed of movement of the endless transfer belt 15 is equal tothe peripheral velocity of the photoreceptor drum 3 and, therefore, norelative movement takes place between the endless transfer belt 15 andthe photoreceptor drum 3.

While the transfer device 11 is so designed as hereinabove described,and when the visible powder image formed on the outer peripheral surfaceof the photoreceptor drum 3 is brought to the primary transfer stationat which that portion of the endless transfer belt 15 is brought intocontact with the photoreceptor drum 3, a primary transfer takes place inwhich the visible powder image is electrostatically transferred onto theendless transfer belt 15 on the basis of the electrostatic chargeimparted by the electrostatic charger 21.

The photoreceptor drum 3 having passed through the primary transferstation is subsequently moved to a cleaning station at which residuetoner and residue electrostatic charge both remaining on thephotoreceptor drum 3 are successively removed by the cleaning unit 22and the main eraser 23 in readiness for the next cycle of copyingoperation.

On the other hand, the toner image transferred onto the endless transferbelt 15 is conveyed together with the endless transfer belt 15 in thedirection shown by the arrow c. It is to be noted that, where themulti-color image is to be formed, the copying operation as hereinabovedescribed is repeatedly executed so that toner images formed indifferent colors can be transferred onto the endless transfer belt 15 inoverlapped fashion with each other.

The copying sheet 100 supplied from the sheet supply device 40 is, afterhaving been timed with the transport of the toner image, passed throughthe timing roller 46 onto the secondary transfer station at which, basedon the discharge effected by the secondary transfer charger 24, asecondary transfer takes place with the toner image consequentlytransferred onto the copying sheet 200.

The copying sheet 100 onto which the toner image has been transferred atthe secondary transfer station is subsequently separated from theendless transfer belt 15 by the separating charger 25 and relayed ontothe endless transport belt 48 by which the copying paper 100 is conveyedto the fixing unit 49. After the toner image on the copying sheet 100has been permanently fixed by the fixing device 49, the copying sheet100 bearing the fixed toner image is discharged onto the receiving tray50.

It is to be noted that residue toner and residue electrostatic chargeboth remaining on the endless transfer belt 15 subsequent to thetransfer of the toner image onto the copying sheet 100 at the secondarytransfer station are successively removed by the cleaning unit 19 andthe charge removing charger 20 in readiness for the next cycle oftransfer operation.

III. Color Discriminating Process and Others

Hereinafter, a color discriminating process will be described.

A color discriminating device 30 comprises a plurality of photoelectricelements (hereinafter referred to as "CCD sensor") 30a arrangedcontinuously in a direction parallel to the axis of rotation of thephotoreceptor drum 3, each of said photoelectric elements 30a having awidth within the range of 1.5 to 2 mm. The CCD sensor 30a is providedwith three photo-diodes LDr, LDg and LDb for detecting three primarycolor components R, G and B included in reflected component of the raysof light which have been radiated to an object, and the color of anon-illuminated object can be discriminated on the basis of respectiveoutputs from the photo-diodes LDr, LDg and LDb.

It is to be noted that the photo-diodes LDr, LDg and LDb are preadjustedto as to give equal outputs by radiating light to objects of the threeprimary colors, that is, red (R), green (G) and blue (B).

The operation associated with the color discrimination is carried outaccording to a flowchart shown in FIG. 3.

Referring now to FIG. 3, when the copying operation describedhereinbefore is initiated followed by the drive of the optical system27, and at step Cl, the CCD sensor 30a detects rays of light reflectedfrom the document placed on the document support 26 and respectiveoutput signals from the photo-diodes LDr, LDg and LDb are inputted asanalog signals. In other words, the color of each of minute areasdelimited by finely dividing the document in two directionsperpendicular to each other can be detected as divided into red (R),green (G) and blue (B) color elements.

Subsequently, the analog signals referred to above are converted intodigital signals fr, fg and fb at step C2, followed by the storage ofthose digital signals fr, fg and fb in a memory A at step C3.

At steps C4 and C5, based on the digital signals fr, fg and fb stored inthe memory A, discrimination coefficient kr', kg' and kb' representativeof respective weights of the red (R), green (G) and blue (B) colors ofan image in each of the minute areas referred to above are calculatedaccording to the following equations.

    kr'=fr/(fr +fg +fb)

    kg'=fg/(fr +fg +fb)

    kb'=fb/(fr +fg +fb)

Also, by comparing a first reference value α with a second referencevalue β which is greater than the first reference value α with respectto each of the color discrimination coefficients kr', kg' and kb' colorconversion is carried out in the following manner to determine colordiscrimination data D (kr, kg, kb).

When α≦ (kr', kg', kb'); kr, kg, kb=2

When β> (kr', kg', kb'); kr, kg, kb=0

When β≦ (kr', kg', kb')<α; kr, kg, kb=1

Then, based on a combination of those color discrimination data D (kr,kg, kb), the color, that is, the reproduced color, of each of the minuteareas is determined in reference to the following Table 1 in such a waythat, if the data represents (2, 2, 0) or (2, 0, 2), it means yellow (Y)or magenta (M), respectively. It is to be noted that in Table 1 thecolor of toner for accomplish the reproduction of each color and thesequence of development thereof are also illustrated.

                  TABLE 1                                                         ______________________________________                                                                      Toner Used and                                                   Reproduced   Developing Sequence                             kr, kg,   kb     Color        Y   M   C   Y   M   BK                          ______________________________________                                        2   2     0      Yellow (Y)       --  --  --  --  --                          2   0     2      Magenta (M)  --      --  --  --  --                          0   2     2      Cyan (C)     --  --      --  --  --                          1   2     0      Light Green (LG)                                                                           --  --          --  --                          0   2     0      Green (G)        --      --  --  --                          2   1     0      Orange (OR)  --      --      --  --                          2   0     0      Red (R)              --  --  --  --                          1   0     2      Violet (V)   --          --  --  --                          0   0     2      Blue (B)     --  --      --      --                          2   2     2      Black or Gray (BK)                                                                         --  --  --  --  --                              ______________________________________                                    

Then, at step C6, where an imaqe editing such as, for example, a maskingin which a portion of the image is removed by an editor or a trimming inwhich a portion of the image is removed is specified, an input of a dataassociated with an area to be edited is accepted, followed by step C7 atwhich the position of the document corresponding to the area to beedited is recognized so that color recognition data correspondingthereto can be rewritten into a predetermined value. Finally, at stepC8, the color discrimination data is stored.

IV. Multi-color Image Forming Method

The operation of the copying machine when the latter is set in amulti-color image forming mode will now be described, first briefly andthen in details.

BRIEF DESCRIPTION OF THE OPERATION

In the case where the copying machine referred to above is used toreproduce an image of eight colors, that is, yellow (Y), magenta (M),cyan (C), light green (LG), green (G), orange (OR), red (R) and black(Bk), first to fifth steps tabulated in Table 2 are sequentiallyexecuted.

It is to be noted that the violet (V) and blue (B) are recognized asmagenta (M) and cyan (C), respectively, and a compromise has been madethat no mixed color of magenta (M) and cyan (C) will not be reproduced.

                  TABLE 2                                                         ______________________________________                                              Developing Image Colors                                                 Steps Toner      Y     M    C   LG   G   OR   R   BK                          ______________________________________                                        1     Y                --   --  --       --       --                          2     M          --         --  --   --           --                          3     C          --    --                --   --  --                          4     Y          --    --   --       --       --  --                          5     BK         --    --   --  --   --  --   --                              ______________________________________                                    

(a) During the first step, electrostatic latent images corresponding toimages of yellow (Y), green (G) and red (R) are formed on the outerperipheral surface of the photoreceptor drum 3, which latent images aresubsequently developed by the yellow toner Ty, the resultant yellowtoner images being subsequently primarily transferred onto the endlesstransfer belt 15.

(b) During the second step, electrostatic latent images corresponding toimages of magenta (M), orange (OR) and red (R) are formed on the outerperipheral surface of the photoreceptor drum 3, which latent images aresubsequently developed by the magenta toner Tm, the resultant magentatoner images being subsequently primarily transferred onto the endlesstransfer belt 15 so as to overlay the yellow toner images which havebeen transferred onto the same transfer belt 15 during the first step.At this time, magenta toner Tm (indicated by in Table 2) correspondingto the image of red (R) is transferred so as to overlay the yellow tonerTy which has been primarily transferred during the first step incorrespondence with the image of red (R). However, the magenta toner Tmwhich is transferred so a to overlay the yellow toner Ty exhibits alower transfer efficiency than that exhibited when transferred onto thetransfer belt which does not bear any toner and, therefore, the amountof adherence of the magenta toner Tm is small.

(c) During the third step, electrostatic latent images corresponding toimages of cyan (C), light green (LG) and green (G) are formed on theouter peripheral surface of the photoreceptor drum 3, which latentimages are subsequently developed by the cyan toner Tc, the resultantcyan toner images being subsequently primarily transferred onto theendless transfer belt 15. At this time, in an image area of green (G),the cyan toner Tc (indicated by in Table 2) is transferred over theyellow toner Ty which has been transferred during the first step. And,the amount of adherence thereof is small.

(d) During the fourth step, electrostatic latent images corresponding toimages of light green (LG) and orange (OR) are formed on the outerperipheral surface of the photoreceptor drum 3, which latent images aresubsequently developed by the yellow toner Ty, the resultant yellowtoner images being subsequently primarily transferred onto the endlesstransfer belt 15. At this time, those toners (indicated by in Table 2)are transferred so as to overlay the cyan toner Tc and the magenta tonerTm which have been transferred respectively during the third and secondsteps. And, the amount of adherence thereof is small.

(e) During the final fifth step, an electrostatic latent imagescorresponding to an image of black (BK) is formed on the outerperipheral surface of the photoreceptor drum 3, which latent image issubsequently developed by the black toner Tbk, the resultant black tonerimage being subsequently primarily transferred onto the endless transferbelt 15.

(f) All of the toners transferred onto the transfer belt 15 in themanner as hereinabove described are transferred onto the copying sheet100 one at a time.

It is to be noted that the overlapping toners transferred onto theendless transfer belt 15 are, when transferred onto the copying sheet100, reversed in position relative to each other. In other words, in thearea corresponding to the image of green (G), the yellow toner Ty andthe cyan toner Tc assume the innermost and outermost positions on theendless transfer belt 15, respectively, however, the yellow toner Ty andthe cyan toner Tc assume the outermost and innermost positions whentransferred from the endless transfer belt 15 onto the copying sheet100.

As for the green image referred to above, almost of all the cyan tonerTc assuming the outermost position on the transfer belt 15 aretransferred onto the copying sheet 100. However, the transfer efficiencyof the yellow toner Ty assuming the innermost position on the transferbelt 15 is considerably lowered and, therefore, the amount of tonertransferred onto the copying sheet 100 becomes smaller than that of thecyan toner.

As a result thereof, at the portion which has been secondarilytransferred, the cyan (C) is conspicuous and can be perceived as green(G).

Conversely, at the area of light green (LG), the yellow (Y) is moreconspicuous than the cyan (C) and can be perceived as light green.

A similar description can apply even in the case of the area of each ofred (R) and orange (OR) and, therefore, the color of the document can berecognized.

DETAILED DESCRIPTION OF THE OPERATION

Hereinafter, the multi-color image forming operation will be describedwith particular reference to FIGS. 4 to 9 and also to FIGS. 10 to 17. Itis to be noted that the process diagrams of FIGS. 11 to 16 areillustrative of the process required to reproduce the colors of thedocument OD having eight-color images of yellow (Y), magenta (M), cyan(C), light green (LG), green (G), orange OR , red (R) and black (Bk).

(i) First step: Sl (See FIG. 4)

When the copying machine is powered and the program starts consequently,the copying machine is initialized at step S101 and the number of copiesdesired to make is set to "1". At subsequent step S102, inputs fromselected one or ones of various keys such as tens keys provided on anoperating panel formed atop the machine housing 1 are accepted.

When a PRINT switch is switched on, the first slider 28, with theilluminator lamp 29 lit, undergoes a pre-scanning movement in thedirection shown by the arrow d to effect a preliminary exposure at stepS103. Rays of light emitted from the illuminator lamp 29 and reflectedfrom the document are detected by the CCD sensor 30a at step S104. Then,based on detection values of the CCD sensor 30a, the previouslydescribed color discriminating process (1) is executed and the color ofeach of the minute areas delimited by dividing the document in the twodirections perpendicular to each other is recognized, the data D1 of thecolor so discriminated being subsequently stored in a memory (steps S104and S105).

Thereafter, at step S106, a decision is made on the basis of thediscriminated color data D1 to determine if the document contains imageareas of yellow (Y), green (G) and red (R). If the result of thedecision at step S106 indicates "No", the second step S2 takes place,but if it indicates "Yes", that is, if the document contains at leastone of the image areas of yellow {Y), green (G) and red (R), anotherdecision is made at step S107 to determine if the document contain onlya monochromatic (single-color) image of yellow (Y). Should the result ofdecision at step S107 indicate that the document contains themonochromatic image of yellow (Y), the monochromatic step SO takesplace, but should it indicate "No", step S108 takes place at which theyellow filter Fy is set as the filter assembly 30 of the optical system27.

It is to be noted that, at step S106, when the copying operation starts,a process is executed with the use of the discriminated color data D1,but when in a multi-copy mode, the process is executed with the use ofdiscriminated color data which have been calculated during the executionof the other steps as will be described later.

When the foregoing operation terminates, the yellow developer unit 7 ofthe developing device 6 is set in position ready to perform thedevelopment at step S109, followed by step S110 at which the outerperipheral surface of the photoreceptor drum 3 is electrostaticallycharged to a predetermined potential. Thereafter, and at step S111, theslider 28 moves with the illuminator lamp 28 lit so that rays of lightradiated to the document and reflected therefrom can be projected ontothe photoreceptor drum 3 through the yellow filter Fy thereby to form anelectrostatic latent image.

At this time, of the rays of light reflected from the document, a lightcomponent radiated to an image colored in a yellow component is absorbedby the yellow filter Fy.

Accordingly, as shown in FIG. 11(a), the rays of light radiated torespective images of yellow (Y), green (G) and red (R) are not exposedon the photoreceptor drum, and respective potentials of portionscorresponding to respective images of yellow (Y), green (G), red (R),light green (LG) and orange (OR) remains at respective values equal tothe potential built up at the time of charging and, therefore,electrostatic latent images Iy, Ig, Ir, Ilg and Ior such as shown inFIG. 11(a) are formed on the photoreceptor drum 3. At the same time, theamount of light reflected from an image of black (Bk) color is small andthe potential of a portion corresponding thereto remains at a valueequal to the potential built up at the time of charging and, therefore,an electrostatic latent image Ibk is also formed on the photoreceptordrum 3.

Also, during the exposure the CCD sensor 30a detects the reflected raysof light and, on the basis of the output from the CCD sensor 30a, acolor discriminating process (2) is executed at step S117 to provide adiscriminated color data D2 of each area which is subsequently stored ina memory at step S118. The results thereof are utilized in the practiceof the second step S2.

Also, around the photoreceptor drum, based on the discriminated colordata D1 or discriminated color data provided by a color discriminatingprocess executed immediately therebefore, the light emitting elements ofthe editing erase 5 are individually controlled to be switched on andoff to leave only the electrostatic latent images Iy, Ig and Ir, but torender the potentials of the other areas to be lowered down to aboutzero volt at step S112. In this way, the potential of that portion ofthe electrostatic latent image Ibk corresponding to the image of black(Bk) is also lowered down to about zero volt.

It is to be noted that, although the color discriminating process (2)and the editing erasure take place substantially at the same time, thediscriminated color data D1 stored in the memory are outputted to theediting eraser 5 before the discriminated color data D2 resulting fromthe color discriminating process (2) are stored, and therefore, theediting eraser 5 will not operate on the basis of the discriminatedcolor data D2.

Thereafter, at step S113, the electrostatic latent images Iy, Ig and Irare supplied with the yellow toner at the developing station from theyellow developer unit 7 to render them to form the yellow toner imagesas shown in FIG. 11(b).

On the other hand, in the transfer device 11, the surface of the endlesstransfer belt 15 is charged by the electrostatic charger 21 to apredetermined potential at step S119 while that portion of the transferbelt 15 between the support rollers 11 and 12 are brought into contactwith the outer peripheral surface of the photoreceptor drum 3 so that,as shown in FIG. 11(c), the yellow toner (Ty) images can be primarilytransferred onto the transfer belt 15 at steps S114 and S120), followedby the second step T2.

The residue toner and the residue electrostatic charge remaining on theouter peripheral surface of the photoreceptor drum 3 are successivelyremoved at steps S115 and S116, followed by the step S2.

On the other hand, at the sheet supply and transport system, on thebasis of respective rotations of the sheet supply roller 44 and/or thetransport roller 45, the copying sheet 100 is transported to a positionpreceding the timing roller 46 at step S121, followed by the second stepP2.

(ii) Second Step: S2 See FIG. 5)

This second step S2 is a process which takes place upon the terminationof the first step Sl or in the event that, as a result of the decisionat step S106 during the execution of the first step Sl, no one of theimages of yellow Y), green (G) and red R colors is found in thedocument.

Referring now to FIG. 5, at step S201, on the basis of the discriminatedcolor data D2 provided as a result of the color discriminating process(2) or the discriminated color data resulting from the colordiscriminating process executed immediately therebefore, a decision ismade to determine if images of the document contain any one of themagenta (M), orange (OR) and red (R) colors. Should the result of thedecision at step S201 indicate "Yes", the next succeeding decision stepS202 is executed, but should it indicates "No", the program skips ontothe third step S3.

During the decision step at S202, a decision is made to determine if thedocument contains only a monochromatic image of magenta (M). If a resultof this decision indicates "Yes", the monochromatic step SO takes place,but if it indicates "No", the magenta filter Fm is set in the path oftravel of the reflected light rays at step S203.

Around the photoreceptor drum 3, the magenta developer unit 8 of thedeveloping device 6 is set in position ready to perform the developmentat step S204, followed by step S205 at which the outer peripheralsurface of the photoreceptor drum 3 is electrostatically charged to apredetermined potential. Thereafter, and at step S206, the opticalsystem 27 is driven to effect the exposure of the image of the documentat step S206 thereby to form the electrostatic latent images Im, Ior, Irand Ibk on the photoreceptor drum 3 as shown in FIG. 12(a). Also, duringthe exposure the CCD sensor 30a detects the reflected rays of light atstep S212 and, on the basis of the output from the CCD sensor 30a, acolor discriminating process (3) is executed at step S213 to providediscriminated color data D3 of each area which is subsequently stored ina memory.

Thereafter, rays of light are radiated from the editing eraser 5 ontothe photoreceptor drum 3 thereby to leave only the electrostatic latentimages Im, Ior and Ir corresponding respectively to the magenta (M),orange (OR) and red (R) images, but to render the potentials of theother areas to be lowered down to about zero volt at step S207.

Accordingly, during the subsequent developing operation, the magentatoner Tm is supplied only to the electrostatic latent images Im, Ior andIr at step S208 as shown in FIG. 12(b).

The magenta toner Tm so supplied onto the electrostatic latent imagesare primarily transferred at steps S209 and S215 onto the endlesstransfer belt 15 which has been charged at step S214 by theelectrostatic charger 21 and onto which the yellow toner Ty has beentransferred as a result of the execution of the first step Sl. Inparticular, at the time of the transfer onto the endless transfer belt15, in the red (R) area, the magenta toner Tm is overlaid on the yellowtoner Ty.

In order to avoid any displacement between the toner image formed withthe yellow toner Ty and the toner image formed with the magenta tonerTm, the timing at which the optical system 27 is operated and the timingat which both of the photoreceptor drum 3 and the transfer belt 15 aredriven are so adjusted as to avoid such displacement. This adjustment iselectrically precisely accomplished.

It is to be noted that, in the transfer belt 15 which has undergone thefirst step Sl, even though the electrostatic charger 21 is stablyoperated, that area of the transfer belt 15 where the yellow toner Tyhas been transferred cannot be electrostatically charged sosatisfactorily as the remaining area of the transfer belt 15 where notoner is transferred. Because of this, in the area where the yellowtoner Ty has been transferred, a satisfactory potential differencecannot be attained relative to the photoreceptor drum 3 so much as thatbetween the photoreceptor drum 3 and the other areas of the transferbelt 15 and, therefore, the toner transfer efficiency tends to belowered. Accordingly, in the image area of red (R), the number of tonerparticles of the magenta toner Tm applied over the yellow toner Ty perunit surface area becomes smaller than that of the yellow toner Ty. SeeFIG. 12(b).

When and after the primary transfer completes in the manner ashereinabove described, the residue toner and the residue electrostaticcharge remaining on the outer peripheral surface of the photoreceptordrum 3 are successively removed at steps S210 and S211, followed by thethird step S3.

On the other hand, at both of the transfer device 11 and the sheetsupply and transport system, on the basis of the discriminated colordata D1 or the discriminated color data resulting from the colordiscriminating process executed immediately therebefore, a decision ismade to determine if the image of the document is colored only in yellow(Y), magenta (M) and red (R). If the result of decision at step S216indicates that the document contains colors other than yellow (Y),magenta (M) and red (R), the respective third steps T3 and P3 take place(Steps S216 and S221).

Also, where the image in the document is colored only in yellow (Y),magenta (M) and red (R), the copying sheet 100 is, after having beensynchronized with the transport of the toner images on the transfer belt15, transported by the timing roller 46 onto the secondary transferstation where the secondary transfer charger 24 is installed at stepS222, and the toner images on the transfer belt 15 are subsequentlysecondarily transferred onto the copying sheet 100 at step S217 andS222.

Then, in the transfer device 11, the residue toner and the residueelectrostatic charge on the transfer belt 15 are removed successively atsteps S218 and S219, followed by Mu if the machine is in a multi-copymode, or followed by the interruption of the operation at step S220 ifthe machine is in a single copy mode. On the other hand, in the sheetsupply and transport system, the copying sheet 100 is supplied onto thetransfer device 49 and, after the toner images referred to above havebeen transferred onto the copying sheet 100, the copying sheet 10bearing the images formed with the use of yellow (Y), magenta (M) andred (R) toners is discharged onto the receiving tray 50 at step S225.However, if the copying machine is set in the multicopy mode asdetermined at a decision step S226, Mu takes place, but if it is set inthe single copy mode, the copying machine is brought to a halt.

(iii) Third Step: S3 (See FIG. 6)

This third step S3 is a process which takes place upon the terminationof the second step S2 or depending on the result of decision performedat step S201 during the execution of the second step S2, and the flow ofthe third step S3 is substantially identical with that of the secondstep S2 except for the difference in contents to be determined.

Referring now to FIG. 6, at step S301, on the basis of the discriminatedcolor data resulting from the color discriminating process executedimmediately therebefore, a decision is made to determine if images ofthe document contain any one of the cyan (C), light green (LG) and green(G). Should the result of the decision at step S301 indicate "Yes", thenext succeeding decision step S302 is executed, but should it indicates"No", the program skips onto the fourth step S4.

During the decision step at S302, a decision is made to determine if thedocument contains only a monochromatic image of cyan (C). If a result ofthis decision indicates "Yes", the monochromatic step SO takes place,but if it indicates "No", the cyan filter Fc is set in the path oftravel of the reflected light rays at step S303.

Around the photoreceptor drum 3, the cyan developer unit 9 of thedeveloping device 6 is set in position ready to perform the developmentat step S304, followed by step S305 at which the outer peripheralsurface of the photoreceptor drum 3 is electrostatically charged to apredetermined potential. Thereafter, and at step S306, the opticalsystem 27 is driven to effect the exposure of the image of the documentat step S306 thereby to form the electrostatic latent images Ic, Ilg, Igand Ibk on the photoreceptor drum 3. Also, during the exposure the CCDsensor 30a detects the reflected rays of light at step S312 and, on thebasis of the output from the CCD sensor 30a, a color discriminatingprocess (4) is executed at step S313 to provide discriminated color dataD4 of each are which is subsequently stored in a memory.

Thereafter, rays of light are radiated from the editing eraser 5 ontothe photoreceptor drum 3 thereby to leave only the electrostatic latentimages Ic, Ilg and Ig corresponding respectively to the cyan (C), lightgreen (LG) and green (G) images, but to render the potentials of theother areas to be lowered down to about zero volt at step S307 as shownin FIG. 13(a). Accordingly, during the subsequent developing operation,the cyan toner Tc is supplied only to the electrostatic latent imagesIc, Ilg and Ig at step S308 as shown in FIG. 13(b). The cyan toner Tc sosupplied onto the electrostatic latent images are primarily transferredat steps S309 and S315 onto the endless transfer belt 15 which has beencharged at step S314 by the electrostatic charger 21 and onto which theyellow toner Ty has been transferred as a result of the execution of thefirst step Sl. In particular, at the time of the transfer onto theendless transfer belt 15, in the portion corresponding to the green (G)image, the cyan toner Tc is overlaid on the yellow toner Ty, however,the mount of transfer thereof is smaller than that of the yellow tonerTy. (See FIG. 13(b)).

When and after the primary transfer completes in the manner ashereinabove described, the residue toner and the residue electrostaticcharge remaining on the outer peripheral surface of the photoreceptordrum 3 are successively removed at steps S310 and S311, followed by thefourth step S4.

On the other hand, at both of the transfer device 11 and the sheetsupply and transport system, on the basis of the discriminated colordata D3, a decision is made to determine if the image of the document iscolored only in cyan (C), light green (LG) and green {G). If the resultof decision indicates that the document contains colors other than cyan(C), light green (LG) and green (G), the fourth steps T3 and P3 takeplace (Steps S316 and S321).

Also, where the image in the document is colored only in yellow (Y),magenta (M), cyan (C), green (G) and red (R), the toner images on thetransfer belt 15 are subsequently secondarily transferred onto thecopying sheet 100 at steps S317 and S322.

Then, in the transfer device 11, the residue toner and the residueelectrostatic charge on the transfer belt 15 are removed successively atsteps S318 and S319, followed by Mu if the machine is in the multi-copymode, or followed by the interruption of the operation at step S320 ifthe machine is in the single copy mode.

On the other hand, in the sheet supply and transport system, the copyingsheet 100 is supplied onto the transfer device 49 and, after the tonerimages referred to above have been transferred onto the copying sheet100, the copying sheet 10 bearing the images formed with the use ofyellow (Y), magenta (M), cyan (C) green (G) and red (R) toners isdischarged onto the receiving tray 50 at step S325. However, if thecopying machine is set in the multicopy mode, Mu takes place, but if itis set in the single copy mode, the copying machine is brought to a halt(steps S324 to S326).

(iv) Fourth Step: S4 (See FIG. 7)

The fourth step S4 is a process which takes place upon the terminationof the third step S3 or which follows from step S301 during theexecution of the third step S3, and contents to be processed during thisfourth step S4 are substantially identical with those during any one ofthe second and third steps.

Referring now to FIG. 7, at step S401, on the basis of the discriminatedcolor data resulting from the color discriminating process executedimmediately therebefore, a decision is made to determine if images ofthe document contain any one of the light green (LG) and green (G).Should the result of the decision at step S401 indicate "Yes", the nextsucceeding step S402 is executed, but should it indicates "No", theprogram skips onto the fifth step S5.

Then at step S402, the yellow filter Fy is set in the path of travel ofthe reflected light rays. Around the photoreceptor drum 3, the yellowdeveloper unit 7 of the developing device 6 is set in position ready toperform the development at step S403, followed by step S404 at which theouter peripheral surface of the photoreceptor drum 3 iselectrostatically charged to a predetermined potential. Thereafter, andat step S405, the electrostatic latent images Iy, Ig, Ir, Ilg, Ior andIbk on the photoreceptor drum 3.

Also, during the exposure the CCD sensor 30a detects the reflected raysof light at step S411 and, on the basis of the output from the CCDsensor 30a, a color discriminating process (5) is executed at step S412to provide discriminated color data D5 of each area which issubsequently stored in a memory.

Thereafter, rays of light are radiated from the editing eraser 5 ontothe photoreceptor drum 3 thereby to leave only the electrostatic latentimages Ilg and Ior corresponding respectively to the light green (LG)and orange (OR) images, but to render the potentials of the other areasto be lowered down to about zero volt at step S406. Accordingly, duringthe subsequent developing operation, the yellow toner Ty is suppliedonly to the electrostatic latent images I(g and Ior at step S407 asshown in FIG. 14(a).

The yellow toner Ty so supplied onto the electrostatic latent images areprimarily transferred at steps S408 and S414 onto the endless transferbelt 15.

In particular, at the time of the transfer onto the endless transferbelt 15, in the portions corresponding to the light green (LG) andorange OR) images, the yellow toner Ty is overlaid on the cyan toner Tcand the magenta toner Tm. However, the mount of transfer thereof issmaller than that of the magenta and cyan toners. (See FIG. 14(b)).

When and after the primary transfer completes in the manner ashereinabove described, the residue toner and the residue electrostaticcharge remaining on the outer peripheral surface of the photoreceptordrum 3 are successively removed at steps S409 and S410, followed by thefifth step S5.

On the other hand, at both of the transfer device 11 and the sheetsupply and transport system, on the basis of the discriminated colordata D4, a decision is made to determine if the image of the document iscolored only in yellow (Y), magenta (M), cyan (C), light green (LG),green (G), orange (OR) and red (R). If the result of decision indicatesthat the document contains colors other than yellow (Y), magenta (M),cyan (C), light green (LG), green (G), orange (OR) and red (R), thefifth steps T4 and P5 take place Steps S415 and S420).

Also, where the image in the document is colored only in yellow (Y) red(R), the toner images on the transfer belt 15 are subsequentlysecondarily transferred onto the copying sheet 100 at steps S416 andS422.

Then, in the transfer device 11, the residue toner and the residueelectrostatic charge on the transfer belt 15 are removed successively,followed by Mu if the machine is in the multi-copy mode, or followed bythe interruption of the operation if the machine is in the single copymode (Steps S417 to S419). On the other hand, in the sheet supply andtransport system, after the toner images referred to above have beentransferred onto the copying sheet 100, the copying sheet 10 bearing theimages formed with the use of yellow (Y), magenta (M), cyan (C), lightgreen (LG), green (G), orange (OR) and red (R) toners is discharged ontothe receiving tray 50 (Steps S423 to 425).

(v) Fifth Step: S5 (See FIG. 8)

The fourth step S5 is a process which takes place upon the terminationof the fourth step S4 or which takes place when the result of decisionat step S401 has indicated that none of the light green (LG) and theorange (OR) is contained in the document.

Referring now to FIG. 8, at step S501, on the basis of the discriminatedcolor data calculated immediately therebefore, a decision is made todetermine if images of the document contain the black (Bk). Should theresult of the decision at step S501 indicate "Yes", the next succeedingstep S502 is executed, but should it indicates "No", a decision stepS503 takes place. If the result of decision at step S502 indicates thatthe image in the document is colored only in black (Bk), themonochromatic step SO takes place, but if it indicate that the image inthe document is not only in black (Bk) the black filter Fbk set in thepath of travel of the reflected light rays at step S504. On the otherhand, if the result of decision at step S503 indicates that the machineis in the multi-copy mode, Mu takes place, but if it indicates that themachine is not in the multicopy mode, that is, the machine is in thesingle copy mode, the operation terminates.

Around the photoreceptor drum 3, the black developer unit Bk of thedeveloping device 6 is set in position ready to perform the developmentat step S505, followed by step S506 at which the outer peripheralsurface of the photoreceptor drum 3 is electrostatically charged to apredetermined potential. Thereafter, and at step S507, the electrostaticlatent images Ibk are formed on the photoreceptor drum 3 (See FIG.15(a)). In parallel therewith, the exposure the CCD sensor 30a detectsthe reflected rays of light at step S514, followed by the execution of acolor discriminating process (6) at step S515 to calculate discriminatedcolor data D6.

Thereafter, rays of light are radiated from the editing eraser 5 ontothe photoreceptor drum 3 thereby to leave only the electrostatic latentimages Ibk corresponding respectively to the black images, but to renderthe potentials of the other areas to be lowered down to about zero voltat step S508. Accordingly, during the subsequent developing operation,the black toner Tbk is supplied only to the electrostatic latent imagesIbk at step S509 as shown in FIG. 15(a). The black toner Tbk so suppliedonto the electrostatic latent images are primarily transferred at stepsS510 and S517 onto the endless transfer belt 15 which has beenelectrostatically charged at step S516 See FIG. 15).

The toners transferred onto the endless transfer belt 15 throughout thefirst to fifth steps in the manner as hereinbefore fully described areall secondarily transferred onto the copying sheet 100 which has beensupplied through the sheet supply and transport system, and the copyingsheet 100 having the eight-color image so formed is subsequentlydischarged onto the receiving tray 50. If the machine is in themulti-copy mode, Mu takes place, but if the machine is in the singlecopy mode, the operation terminates. See steps S518 and S523 to S526).

It is to be noted that the residue toner and the residue electrostaticcharge on the transfer belt 15 are removed successively in readiness forthe next cycle, followed by Mu if the machine is in the multi-copy mode,or followed by the interruption of the operation if the machine is inthe single copy mode (Steps S511 to S513 and S519 to S521).

After the foregoing process, in each area where a plurality of thetoners of different colors have been transferred, the toner occupyingthe innermost position on the transfer belt 15 appears at the outermostposition when transferred onto the copying sheet.

(vi) Monochromatic Step: SO (See FIG. 9)

This monochromatic step SO takes place when during the execution of anyone of the first to fifth steps S1 to S5 the document has beendetermined as containing only the monochromatic image of one of yellow(Y), magenta (M), cyan (C) and black (BK) color.

During the execution of this monochromatic step SO, the filtercorresponding to the color of the document image is selected andinterposed in the path of travel of the reflected light rays at stepS10. More specifically, where the document image is in yellow, magenta,cyan or black color, the yellow, magenta, cyan or black filter Fy, Fm,Fc or Bk is selected.

Then, around the photoreceptor drum, one of the developer units whichcontain the toner of a color identical with the color of the documentimage is set in position to apply the toner onto the photoreceptor drumwhich has been electrostatically charged to a predetermined potential,followed by the exposure through the optical system 27 to form thecorresponding electrostatic latent image which is subsequently renderedto be a visible toner image of the predetermined color (See steps S20 toS23.).

The toner image is thereafter primarily transferred onto the endlesstransfer belt which has been electrostatically charged (See steps S24,S30 and S31.).

The toner which has been transferred onto the transfer belt 15 issecondarily transferred onto the copying sheet 100 which has beensupplied from the sheet supply device 40, which sheet 100 issubsequently discharged onto the receiving tray 50 after having passedthrough the fixing device 49 (See steps S32 and S40 to S42).

The residue toner and the residue electrostatic charge on thephotoreceptor drum 3 and the transfer belt 15 are subsequently removedin readiness for the respective next succeeding cycle of operation, and,if the machine is set in the multi-copy mode, the monochromatic step SOis again executed (See steps S25, S26, S33, S34 and S50).

From the foregoing description of the present invention made inconnection with the preferred embodiment thereof, it is clear that, bycombining the yellow and cyan toners together, an image of light greenor green color which is an intermediate color of them can be obtained,and by combining the yellow and magenta toners, an image of red ororange which is an intermediate color of them can also be obtained. Inother words the use of the toners of three different colors issufficient and effective to enable a reproduction of images of sevendifferent colors.

Accordingly, the present invention makes it possible for the imageforming machine to be provided with three developer units foraccommodating yellow, cyan and magenta toners so that the images ofseven different colors can be copied on the copying sheet. Therefore,the resultant image forming machine based on the present invention isvery compact.

Also, since all of the toner images on the endless transfer belt aretransferred onto the copying sheet at the same time, the image free fromcolor deviation can be advantageously obtained.

Although the present invention has been fully described in connectionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be noted that various changes and modifications areapparent to those skilled in the art. By way of example, although indescribing the preferred embodiment reference has been made to the imagecolored in eight different colors, the reproduction of the image in sixdifferent colors, for example, yellow (Y), magenta (M), cyan (C), green(G), red (R) and black (Bk), is possible if the second step shown inTable 2 is omitted.

Also, intermediate colors formed by mixing yellow (Y), magenta (M), cyan(C), green (G), red (R) and black (Bk) are not always limited to thosedescribed in connection with the preferred embodiment.

Accordingly, such changes and modifications are to be understood asincluded within the scope of the present invention as defined by theappended claims unless they depart therefrom.

What is claimed is:
 1. An image forming method for forming a toner imageon a recording medium from an original image, which comprises the stepsof:reading color data from an original on a picture element to pictureelement basis with the use of an original image reading means;calculating magenta and cyan components on the basis of said color dataread by said reading means to produce color discriminating datanecessary to determine which toner to use; changing said colordiscriminating data so as to select either magenta or cyan colorswithout using both magenta and cyan toners in the event that said colordiscriminating data contains both magenta and cyan components; andforming said toner image on said recording medium based upon the changedcolor discriminating data.
 2. The method as claimed in claim 1, whereinan electrostatic latent image forming means forms an electrostaticlatent image on an electrostatic latent image carrier based upon thechanged color discriminating data and developing said electrostaticlatent image with a toner determined by said color discriminating data.3. The method as claimed in claim 2, wherein said electrostatic latentimage forming step includes a step of imparting an electrostatic chargeuniformly over said electrostatic latent image carrier having aphotoelectrically conductive layer, a step of projecting imagewise lightof the original image onto said electrostatically charged electrostaticlatent image carrier through a filter capable of passing therethroughrays of light of a color other than the color of toner determined bysaid color discriminating data determined during said calculating step,whereby said electrostatic latent image of a color corresponding to thecolor of toner determined by said color discriminating data determined,during said calculating step, can be formed on said electrostatic latentimage carrier, and a step of exposing said electrostatic latent image,so formed, through an array of light emitting elements, said lightemitting element being lit at a portion other than a portioncorresponding to said color discriminating data changed during saidchanging step.
 4. An image forming method for forming a toner image on arecording medium from an original image, which comprises the stepsof:reading color data using an original image reading means; calculatingmagenta and cyan components on the basis of said color data ready bysaid reading means to produce color discriminating data necessary todetermine which toner to use; changing said color discriminating data soas to select either a magenta or cyan color without using both magentaand cyan toners in the event that said color discriminating datacontains both magenta and cyan components; forming an electrostaticlatent image on an electrostatic latent image carrier by means of anelectrostatic latent image forming means on the basis of said changedcolor discriminating data; developing said electrostatic latent image soformed with a toner determined by said color discriminating data to forma toner image on said electrostatic latent image carrier; transferringsaid toner image onto an intermediate transfer medium; saidelectrostatic latent image forming step, said developing step and saidtransfer step being sequentially repeated a predetermined number oftimes to form said toner image on said intermediate transfer medium; andtransferring onto a recording medium all said toner images formed byyellow, magenta or cyan toner on said intermediate transfer medium.
 5. Amethod for forming a color image by forming a yellow, magenta or cyantoner image on a recording medium based upon an original image, whichcomprises the steps of:reading color data using an original imagereading means; calculating yellow, magenta and cyan components on thebasis of said color data read by said reading means to produce colordiscriminating data necessary to determine which toner to use; changingsaid color discriminating data so as to select either magenta or cyancolor without using both magenta and cyan toners in the event that saidcolor discriminating data contains both magenta and cyan components;determining a sequence of use of a yellow toner according to said colordata in the event it is determined that said color discriminating datacontains yellow and magenta components or in the event it is determinedthat said color discriminating data contains yellow and cyan components;and forming said toner image on said recording medium by forming,according to said sequence of use, a yellow, magenta or cyan toner imagebased upon said changed color discriminating data.
 6. The method asclaimed in claim 5, wherein said toner image forming step includes astep for forming said toner image on an intermediate transfer medium bysequentially forming, according to a predetermined sequence of transfer,a yellow, magenta or cyan toner image based upon said changed colordiscriminating data and a step for transferring onto said recordingmedium all of said toner images so formed on said intermediate transfermedium.